The ISO standard allows the film speed to be test by either the ASA/BS system (which Kodak had relaxed) or the more practical DIN system used by Agfa.
Originally Posted by MattKing
So with Agfa films like APX100 where the speed was tested using the DIN standard my Zone system tests gave me an EI thge same as the ISO speed, and many others found the same. The opposite was true with Tmax100 where I needed to use50 EI instead of the box 100 ISO.
It's also worth remembering that the developer used for the ASA tests gave slightly better film speed and shadow detail than D76.
While I appreciate and understand all the sensitometry data involved here, I really believe the answer to Bill's original question can be expressed in less technical terms.
First, we haven't addressed one important factor in the ZS; that of adjusting development time to include a wide tonal range of 9 Zones (or 10, depending on how you're counting) for "Normal." This, I believe, is somewhat more range than the ISO tests result in. Therefore, a ZS N-development is usually less than the ISO standard development. This results in a skosh less effective film speed.
Secondly, we Zonies use our spot meters as arbitrary tools to place shadow values and see where other values fall. In practice, this ends up having very little or nothing at all to do with speed points or metering points (or middle grey for that matter). It is more an "I know what my shadows will look like when I place them in Zone III (or II or IV), because I've tested, and that's where I want them" thing. That coupled with "and now I have to adjust development a bit to get the highlight values where I want them" (many times callously disregarding the slight changes in effective film speed with shortened or lengthened development!) and we can consistently get negatives that are "in the ballpark" enough to be able to be printed well, even if we need to print a grade higher or lower than our target grade. For me, using the meter to help me visualize print values is paramount.
Furthermore, I think the Zone System emphasis on shadow detail and shadow metering, coupled with the relatively greater range of Zones and the resulting need to develop a bit less results in E.Is. that are slower than box speed for two reasons. First, is the slight bit of film speed loss with reduced development to accommodate more Zones together with the use of compensating developers/techniques, which also results in film speed loss. Then there is a tendency to place of the shadow values higher than might be done in the ISO standard in order to get "detail." Since no one takes up the Zone System without having a desire for richly-detailed shadows, and since most of them are shooting large-format film and aren't overly concerned about grain, there is a tendency in the entire process of determining a personal E.I. and then metering and placing shadows to err on the side of overexposure.
I know that when I test for E.I. and there is any doubt at all exactly where that Zone I density falls, I'll just rate the film a third-stop slower. What the heck, one third of a stop makes very little difference in the field. Similarly, if I'm not sure exactly where I want a shadow to go, I'll err on the side of overexposure.
In the end, a personal E.I. is not a film-speed determination, but an accommodation and adjustment of all elements of one's photo-making procedure in order to get repeatable and usable results in rendering of shadow detail. It only stands to reason that, consciously or unconsciously, we tend to err on the side of overexposing, thus building safety factors into our systems without even being aware of it.
FWIW, I shoot 320 Tri-X at E.I. 250 and TMY at E.I. 320, both only a third of a stop slower than box speed. This with PMK. So not all of us Zonies end up rating our film at half box speed. Even so, I'm aware that I've built maybe 2/3 of a stop cushion into my system. When the chips are down and I need a faster shutter speed to stop wind movement or the like, I won't hesitate to go ahead and "underexpose" my film by a stop (thus undercutting my safety factor). No one likes printing underexposed negs, but sometimes that's the only way to get a shot...
I assume you mean "no safety factor" and that makes sense.
Originally Posted by Stephen Benskin
Part of what I see running through ZS thought, discussion, and testing is that everybody's seems to be trying to balance the absolute minimum amount of exposure (so as to minimize grain, exposure time, etc...) against a loss of expected shadow detail. They want to know where the edge of the cliff is.
Many times though it seems that the generic "1/2 box speed" advice is a lot like a parent telling a child to "step back from the edge of that cliff" even though we know its not really a cliff, the slope is just changing; it's not an absolute point beyond which things are unworkable, it's just starting to require more work.
It seems to me that over the years also, lenses/coatings had improved and flare was less an issue, at least in part that allowed the switch to ISO from ASA on a technical basis.
No, it has no exposure constant.
Originally Posted by markbarendt
The edge of the cliff is the fractional gradient point. That is why it was chosen as the speed point. Once again, just because speed is calculated from the speed point doesn't mean that the shadow exposure is designed to fall there. That's where the exposure constant comes in. Post #23 and #25 on the thread ISO Speed Determination Constants begins to discuss this issue. I've also offered to email Simple Method for Approximating the Fractional Gradient Speed of Photographic Materials to anyone interested. Plus in thread #80, I've uploaded a pdf copy of Safety Factors in Camera Exposure if anyone is interested on exposure placement in regards to film speed. Thread #81 has a copy of Calibration Levels for Film and Exposure Devices if you want to know about the actual relationship between the metered exposure and speed point.
Flare is less of an issue. The illuminance range of the camera image has increased from approximately 1.50 to 1.80. But it is still a factor and many discrepancies between methodologies or confusion over numbers not matching up is do to flare.
Film speed is based on the first excellent print test. It showed that additional exposure over the first excellent print point showed no significant change in quality over a number of stops (large format). Adding 1/3 to 1 stop, therefore, isn't a big deal. Remember, film speeds were a stop slower before 1960.
The Zone System doesn't have any great insight into the photographic process. It's just that many people aren't familiar with anything else. The Zone System is just a simplified version of tone reproduction theory. Under tone reproduction the statistically average scene luminance range is 2.20. This is based on diffused highlights and shadow with some detail. That doesn't mean it ignores accent black and specular highlights. These simply print outside the LER range of the paper. Adams isn't much different. Anyone remember the aim density range in the testing section? It's from Zone I to Zone VIII, seven stops.
And I'm sorry Ian, you really should support such claims.
So Stephen meant there is no exposure constant (0.8 in the numerator of the speed formula). But you are also right, there is no safety factor.
Also there is no K. Flare is included. Many of the factors are merged into the procedure and cannot be broken out individually.
Now you're talking my language! We need the technical, but it also needs to be translated to non-technical.
I forgot to mention... If you test with Tungsten lighting, and your meter is overly sensitive to red wavelengths... While your film is "more or less" sensitive to near infrared... Then your "speed" will be lower than ISO [meter needle is higher than it should for the actinic light] and the Tungsten "speed" difference from Daylight may vary between different films of the same ISO rating. This is because meter spectral response may depart from film spectral response. The Zone VI modified spotmeters are intended to correlate well to Tri-X - so if you have one of those, then your tests may correlate better. But change film to a tabular grain type film and the correlation may not be the same. I've tested 400 TMAX and find it can take a very long exposure to a near infrared (visible to the eye) red LED with little fog.
It is worth noting many ZS testers just try to find a net D of 0.1 4 stops below metered without giving more thought to local contrast in the toe. 0.1 above B+F in the context of the ZS is a means to an end, the end being adequate shadow contrast, which is subjective to some extent. A target Zone I density in and of itself is meaningless.
This thread makes me wonder if the 18% grey card issue is really the source of the apparent problem.
We have heard many times about how midtone does not get truly exposed to be mid-tone unless adjusting up the 18% grey card reading per Kodak's instructions. Put in other terms, reading a 12% grey card probably puts midtone closer to middle of the range!
That's a very important fact. The 0.1 Density for Zone I came later. First it was a noticeable difference between blackest black and first detectable tone. Ralph Lambrecht pointed out in the Focal Encyclopedia that later papers were able to reach such a higher D-Max that even the earlier visual match test became unreliable.
Originally Posted by Michael R 1974
If you meter a gray card and attempt to make its tone appear 0.1 density on the negative (or visual match in shadows), then you aren't using it as anything but an arbitrary gray.
Originally Posted by wiltw
The 18% gray issue isn't really a big problem for Zone System tests, though it is an important issue relevant to other metering and exposure topics.